Project Abstract Animal development is remarkably robust to variation, be it of genetic, environmental, or stochastic origin. It is widely believed that complex systems have evolved to buffer against variation because the consequences of uncompensated variation are severely deleterious to the organism, as in the myriad human congenital disorders that arise because of genetic or environmental perturbations in development. Despite the importance of the systems that make development robust, they remain poorly understood. Little is known, for example, about relationship between the level of robustness and the severity of the phenotype were the robustness to fail. Arguably the most common form of large-scale genetic variation encountered in animal development is sex chromosome dosage. In my research, I use the consequences of and response to differences in sex chromosome dose between males and females of Drosophila melanogaster as a general model for understanding how variation in the gene dose can affect development. I recently developed methods to sequence the mRNA from single Drosophila embryos, which allowed me to discover that many X-linked genes that play an important role in patterning the Drosophila embryo are expressed at nearly identical levels before the canonical Drosophila dosage compensation system is activated. This demonstrated the existence of an uncharacterized, yet widely used, system of dosage compensation in the early embryo. Moving forward, I will A.1) determine the mechanism of early zygotic dosage compensation, A.2) characterize dosage compensation in a species with recently derived sex chromosomes to identify genes with a strong requirement for compensation, and A.3) manipulate gene dose in transgenic D. melanogaster to characterize how variation in early development affects adult phenotypes. The last aim will allow dissection of how variation is propagated or suppressed during development, and form the basis of the independent stage of my research program.